... and is a rawafid Shiah a Muslim, and can he lead a prayer, and or can he
stand among us in a prayer

Anyone can stand among us in a prayer provided its outward requirements are
kept - perhaps Allah will lead him to repent through it - but he cannot lead
it.

At the same time, Muslims are obliged to avoid and shun the company of
innovators as in al-Nawawi's Maqasid. On the first question see the post
"Re: Ithnaa Asharis? [updated]" of 23 March 2001 on soc.religion.islam and
the msa-ec archives at sunnah.org. And Allah knows best.